What is MP4?

The container video actually travels in: what phones record, what messaging apps accept, what every player since the iPod era opens. It usually carries H.264 video and AAC audio. Its one structural quirk matters to converters - the index of every frame lives in a table that has to be exactly right, or nothing plays.

What is Opus?

The best-sounding lossy codec at low bitrates by a comfortable margin, which is why it carries the audio in nearly every voice and video call you make. Royalty-free, equally good on speech and music, and usually wrapped in an Ogg container. It always runs at 48kHz internally.